Port doc generator to python 3

Summary: Python 2 is now in end of life. Port to python 3

Test Plan: Succesfull generated en,fr,uk for project calligra and kdegraphics

Reviewers: ltoscano, yurchor, adrianchavesfernandez

Reviewed By: ltoscano, yurchor, adrianchavesfernandez

Subscribers: yurchor, kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D25928